Lyme disease has reached epidemic proportions and continues to spread in certain locations in the northeastern U.S. This region is affected by an especially inflammatory B. burgdorferi genotype, RST1 (OspC Type A) which has been shown to have a high transmission frequency among ticks, and may be primarily responsible for the emergence of Lyme disease in epidemic form in the northeastern U.S. in the late 20th century. However, B. burgdorferi spirochetes do not contain any known toxins that cause virulence. Rather, the disease that ensues following infection is related to the type of inflammatory immune responses that are stimulated by the spirochetes, and that are further augmented by a single nucleotide polymorphism in the TLR1 (1805GG) gene which is present in about half of the European Caucasians. To assess the role of these microbial and host factors in innate immune responses in Lyme arthritis we propose: 1. to compare cytokine and chemokine responses in macrophages, a single cell type, from normal donors with or without the 1805GG polymorphism, stimulated with a highly inflammatory RST1 (OspC type A) strain or a less inflammatory RST2 (OspC type K) strain, using bead-based Luminex assays, 2. to identify the signal transduction networks used by macrophages to sense and respond to OspC type A or K strains and to assess how the 1805GG TLR1 polymorphism alters these networks, by measuring the activation of cellular genes using genome-wide microarrays, and 3. to discern the functional consequences of pathways identified by gene microarrays using RNA interference reagents to inhibit candidate genes by which the TLR1 1805GG polymorphism alters the inflammatory response in our macrophage culture system. Antibiotic-refractory Lyme arthritis may be, at least in part, a syndrome of inappropriate macrophage activation involving both microbial and host genetics. Macrophages are also implicated in the pathogenesis of other rheumatic diseases including RA in which the synovial lesions are similar to those in patients with antibiotic-refractory Lyme arthritis. Thus, it is likely that the targeted approach developed here, to delineate mechanisms underlying inappropriate macrophage activation in antibiotic-refractory Lyme arthritis, will be valuable in understanding how innate immune inflammatory responses may lead to development of autoimmunity in the joint in other inflammatory and rheumatic diseases, including RA.